1. Field of the Invention
The present invention relates to a control system of a vehicle having a continuously variable transmission capable of changing a gear ratio continuously and, more particularly, to a system for executing a control of suppressing the longitudinal vibration surges of a vehicle body when a driving force changes.
2. Related Art
It is known in the vehicle having the continuously variable transmission that the longitudinal vibration surges of the vehicle body occur at the time of ending a shift when a gear ratio is raised on the basis of either a demand for an acceleration or a demand for raising an output, as effected by depressing an accelerator pedal. This phenomenon is called the "backing" or "transient surge", as caused by an inertial torque or the elasticity of a power transmission line. When the gear ratio is raised for a demand for raising the output of a power source such as an engine or a demand for the acceleration, more specifically, the speed of a rotary member to participate in the transmission of the power changes to establish an inertial torque according to the change (in terms of an angular acceleration) of the rotating speed or to the inertial moment. When the shift is ended so that the speed of the rotary member is stabilized to a target speed, moreover, the inertial torque is released, as called so. As a result, the driving torque is temporarily raised by the inertial torque so that the longitudinal vibrations are caused by the torsional elasticity of the power transmission line.
A system for suppressing the vehicular longitudinal vibrations of that kind is disclosed in Japanese Patent Laid-Open No. 8-177997(JPA 8-177997). The system disclosed is constructed such that the vehicular longitudinal vibrations are suppressed by controlling the gear ratio. In this system, the shift ending time is determined on the basis of a target gear ratio and a actual gear ratio, and the half period of the vehicular longitudinal vibrations, as anticipated to occur at the shift ending time, is predicted on the basis of both an input torque to the transmission and an actual gear ratio. Moreover, the system is constructed such that the gear ratio is forcibly corrected to a higher speed side at an instant one half period of the vehicular longitudinal vibrations before the shift ending instant. With this control, therefore, about one half of the inertial torque is released at the instant one half period before the shift ending instant so that the peak of the accompanying vibrations coincides substantially with the shift ending instant. When the torque by the vibrations then lowers, the remaining inertial torque is released so that it acts to suppress the vibrations.
Here, the vehicular longitudinal vibrations may occur not only when the driving force changes but also when the output torque of the power source such as the engine is raised not only by changing the driving force by the inertial torque at the shift ending time but also by an accelerating operation, as effected by depressing the accelerator pedal. Since the aforementioned control system of the prior art is constructed to control the gear ratio only at the shift ending time, however, the vehicular longitudinal vibrations at the acceleration starting time cannot always be suppressed. On the other hand, the control system controls the gear ratio of the continuously variable transmission so as to suppress the vehicular longitudinal vibrations, but the control responsiveness at the time of controlling the gear ratio of the continuously variable transmission is not always higher than the control responsiveness of the output of the power source. As a result, the vibrations may occur due to the delay in the response, or the control for eliminating this may be complicated.
Here, the above-specified Laid-Open describes the reduction in the engine torque in addition to the forced control of the gear ratio to the higher speed side as the control for suppressing the vehicular longitudinal vibrations at the shift ending time. However, the control of the engine torque aids in the control of suppressing the vehicular longitudinal vibrations with the control of the gear ratio. After all, therefore, the vehicular longitudinal vibrations may not be suppressed sufficiently or reliably due to the delay in the control of the gear ratio.